{ "data": { "question": { "questionId": "1820", "questionFrontendId": "1719", "categoryTitle": "Algorithms", "boundTopicId": 551391, "title": "Number Of Ways To Reconstruct A Tree", "titleSlug": "number-of-ways-to-reconstruct-a-tree", "content": "

You are given an array pairs, where pairs[i] = [xi, yi], and:

\n\n\n\n

Let ways be the number of rooted trees that satisfy the following conditions:

\n\n\n\n

Two ways are considered to be different if there is at least one node that has different parents in both ways.

\n\n

Return:

\n\n\n\n

A rooted tree is a tree that has a single root node, and all edges are oriented to be outgoing from the root.

\n\n

An ancestor of a node is any node on the path from the root to that node (excluding the node itself). The root has no ancestors.

\n\n

 

\n

Example 1:

\n\n
\nInput: pairs = [[1,2],[2,3]]\nOutput: 1\nExplanation: There is exactly one valid rooted tree, which is shown in the above figure.\n
\n\n

Example 2:

\n\"\"\n
\nInput: pairs = [[1,2],[2,3],[1,3]]\nOutput: 2\nExplanation: There are multiple valid rooted trees. Three of them are shown in the above figures.\n
\n\n

Example 3:

\n\n
\nInput: pairs = [[1,2],[2,3],[2,4],[1,5]]\nOutput: 0\nExplanation: There are no valid rooted trees.
\n\n

 

\n

Constraints:

\n\n\n", "translatedTitle": "重构一棵树的方案数", "translatedContent": "

给你一个数组 pairs ，其中 pairs[i] = [xi, yi] ，并且满足：

\n\n\n\n

令 ways 为满足下面条件的有根树的方案数：

\n\n\n\n

两棵树被视为不同的方案当存在至少一个节点在两棵树中有不同的父节点。

\n\n

请你返回：

\n\n\n\n

一棵 有根树 指的是只有一个根节点的树，所有边都是从根往外的方向。

\n\n

我们称从根到一个节点路径上的任意一个节点（除去节点本身）都是该节点的 祖先 。根节点没有祖先。

\n\n

 

\n\n

示例 1：

\n\n
\n输入：pairs = [[1,2],[2,3]]\n输出：1\n解释：如上图所示，有且只有一个符合规定的有根树。\n
\n\n

示例 2：

\n\"\"\n
\n输入：pairs = [[1,2],[2,3],[1,3]]\n输出：2\n解释：有多个符合规定的有根树，其中三个如上图所示。\n
\n\n

示例 3：

\n\n
\n输入：pairs = [[1,2],[2,3],[2,4],[1,5]]\n输出：0\n解释：没有符合规定的有根树。
\n\n

 

\n\n

提示：

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